Electronic circuits comprise a plurality of operating electronic components, typically including transistors, resistors, delays and integrated circuits ("ICs") or chips. Such ICs may include ROMS, memory chips, drivers, receivers, gates and microprocessors. ICs serve a variety of tasks and may be classed on the basis of their complexity such as being small scale integration ("SSI"), medium scale integration ("MSI"), large scale integration ("LSI") or very large scale integration ("VLSI") type ICs depending on the relative complexity of the chip. The speed at which an IC processes signals may vary between direct current (D.C.) levels and frequencies of several megahertz and higher. The various electrical parameters such as voltage and current are fixed during manufacture and are generally described as a range in the manufacturers specifications by way of a defined minimum, maximum and typical. The waveforms processed by an IC may be classified as either digital or analog. Analog waveforms may take any shape while digital signals are ideally square waves. Various factors, including the functional integrity of the electronic component, may prevent the appearance of a true square wave. Each of the integrated circuits is required to operate normally in order to maintain the operating characteristics of the electronic circuit as a whole.
When the operation of a component on the board is deficient, the operating characteristics of the board change and if the change is of a sufficient magnitude, the operating characteristics will be deficient leading to a breakdown of the desired electronic response of the circuit as a whole.
Determining the source of electronic circuit problems has been difficult. Typically, the source is a deficient component but determining which component is at fault on the board has been a laborious task. Diagnostic tools previously used are sophisticated, difficult to operate, usually non-portable and expensive. In field repair situations, an emphasis may be on avoiding downtime and a field engineer is under pressure to get the system operational without delay. There is a strong tendency, therefore, to swap out boards or subsystems on a probability basis, leaving the task of actual fault identification to the board repair facility. Upon arrival at a board repair depot, the boards are screened and the faults duplicated before any repair action is undertaken. This procedure can be time consuming, expensive, and cause fluctuations in inventory levels of spare boards which makes "just-in-time" management principles difficult to implement.
The test technique of thermal image analysis is an industry accepted method of detecting electronic components with abnormal thermal characteristics. For proven circuit designs, such defects are generally indicative of IC failure or board artwork defects while in new designs the technique can be used to locate underspecified components or poorly designed ventilation.
The prediction of the thermal activity of electronic components involves complex calculations involving numerous variables and parameters, many of which may not be easily measured or predicted themselves. While mathematical models have been constructed to describe electronic components in a laboratory environment under controlled conditions, no practical universal formula can be presented to predict the operating temperature of an IC under all circumstances. Generally, predictions regarding thermal activity must be derived from extrapolation of the data obtained by sampling good components operating under a variety of fixed conditions.
For a certain amount of D.C. power dissipated in a semiconductor, the junction temperature reaches a value determined by such factors as the thermal conductivity of the chip carrier materials and the differential temperature of the environment. As continued in the Motorola Application Note An-509, the junction temperature may be calculated for a steady state condition by the following formula: EQU T(j)=P(d)*R(stdy)+T(amb)
Where:
T(j)=junction temperature PA1 P(d)=power dissipated at the junction PA1 R(stdy)=steady state thermal resistance-junction to ambient PA1 T(amb)=ambient temperature PA1 T(jav)=average junction temperature increase PA1 T(c)=chip case temperature PA1 R(jc)=thermal resistance-junction to case PA1 P(d)=DC power PA1 D=duty cycle
The above equation holds true only for DC power at thermal equilibrium. Under dynamic conditions, the thermal response of the system must also be taken into account and duty cycle analysis must be performed. The junction temperature at the end of a pulse train will not equal the sum of the average temperature rise values because cooling occurs between pulses. Thus, the following equation is applicable: EQU T(jav)-T(c)=R(jc)*P(d)*D
Where:
To further complicate matters, the concept of duty cycle is based on the presence of a steady pulse train of identical square waves, a condition not often seen in practical electronic circuits. Most waveforms are somewhat non-rectangular having varying frequencies and periods.
Of the various damage mechanisms operating on electronic components, many have been found to be linked to power density fluctuations, either as cause or effect. By virtue of the above equations, such damage mechanisms are ultimately tied to temperature variations. Thus, when the operation of an electronic component becomes deficient by any mechanism which alters the net level of power dissipation and such deficiency is of sufficient magnitude and duration as to alter the temperature of the outer surface of the component, then the temperature change recorded on the outer surface will yield a relative measurement of the deficiency. Further, the elevated temperature may itself constitute a damage mechanism causing secondary breakdown.